Hydrocarbons can catalytically be converted with steam to obtain synthesis gas, i.e. mixtures of hydrogen (H2) and carbon monoxide (CO). As is explained in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release and 6th edition 2003, keyword “Gas Production”, this so-called steam reformation (steam reforming) is the most frequently used method for the production of synthesis gas, which subsequently can be converted to further important basic chemicals such as methanol or ammonia. Although it is possible to convert different hydrocarbons, such as for example naphtha, liquefied gas or refinery gases, the steam reformation of methane-containing natural gas (Steam Methane Reforming, SMR) is dominant. The same proceeds strongly endothermally. It is therefore carried out in a reformer furnace in which numerous catalyst-containing reformer tubes are arranged in parallel, in which the steam reforming reaction takes place. The outer walls of the reformer furnace as well as its ceiling and its bottom are lined or covered with several layers of refractory material which withstands temperatures up to 1200° C. The reformer tubes mostly are fired by means of burners, which are mounted on the upper side or bottom side or at the side walls of the reformer furnace and directly fire the space between the reformer tubes. The heat transfer to the reformer tubes is effected by thermal radiation and convective heat transfer from the hot flue gases.
After preheating by heat exchangers or fired heaters to about 500° C., the hydrocarbon-steam mixture enters into the reformer tubes after final heating to about 500 to 800° C. and is converted there at the reforming catalyst to obtain carbon monoxide and hydrogen. Nickel-based reforming catalysts are widely used. While higher hydrocarbons are completely converted to carbon monoxide and hydrogen, a partial conversion usually is effected in the case of methane. The composition of the product gas is determined by the reaction equilibrium; beside carbon monoxide and hydrogen, the product gas therefore also contains carbon dioxide, non-converted methane and steam.
Another frequently used reforming method is the so-called autothermal reformation (ATR), which represents a combination of steam reformation and partial oxidation, in order to optimize the efficiency. In principle, any hydrocarbon or any hydrocarbon mixture can be used as feed-stock. In the ATR, the steam reformation and the partial oxidation are combined with each other such that the advantage of the oxidation (provision of thermal energy) optimally complements the advantage of the steam reformation (higher hydrogen yield). This is accomplished by an exact dosage of the air and steam supply. The catalysts used here must satisfy particularly high requirements, as they must promote both the steam reformation with the water-gas shift reaction and the partial oxidation. The partial oxidation is effected by controlled combustion of a part of the feedstocks in a burner arranged at the entrance into the autothermal reformer, whereby the thermal energy required for the succeeding steam reformation also is provided.
Both reforming methods, i.e. the steam reformation and the autothermal reformation, can also be used in combination (Combined Reforming).
For energy optimization and/or for feedstocks with higher hydrocarbons, a so-called pre-reformer can be provided upstream of the above-described reforming methods for pre-cracking the feed-stock. Pre-reformation (pre-reforming) mostly is understood to be the application of a low-temperature reforming step, which is arranged upstream of a conventional main reformer, for example a steam reformer, which is operated with natural gas. In contrast to the steam reforming reaction, the reaction equilibrium is set at far lower temperatures during the pre-reformation. The main feature of the pre-reformation is the irreversible, complete conversion of the higher hydrocarbons in the feed mixture to obtain methane and in part synthesis gas constituents. Due to the considerably lower temperature as compared to steam reforming, the main product of the pre-reformation is methane beside non-converted steam. The remaining gas components are hydrogen, carbon dioxide, traces of carbon monoxide and inert components which have already been present in the feedstock. Since virtually all higher hydrocarbons which are present in the natural gas used as feed are converted to methane and synthesis gas constituents, the risk of the formation of coke deposits in the main reformer, which with respect to the operation of the main reformer represents a particularly critical point, is reduced considerably. This permits the decrease of the steam/carbon ratio (S/C) and the increase of the heat load of the reformer tubes, which leads to a generally lower energy consumption and to a reduction in size of the used apparatuses. In addition, an amount of hydrogen already is produced in the pre-reformer by conversion of natural gas, and traces of catalyst poisons left in the feed mixture are adsorbed or absorbed on the pre-reforming catalyst. This leads to the fact that the reforming catalyst present in the main reformer operates under optimum conditions in particular at its inlet.
Upstream of the pre-reforming stage, a desulfurization stage mostly is provided, in order to remove sulfur components of the feedstock, which act as catalyst poison for the catalysts contained in the downstream reformers. The desulfurization can be effected purely by adsorption, for example on adsorbents on the basis of zinc oxide. For some applications, the hydrogenating desulfurization is preferred, in which the sulfur bound in organic and inorganic sulfur components is released in the presence of suitable catalysts by means of hydrogen in the form of hydrogen sulfide and subsequently is bound to adsorbents as described above. Therefore, said desulfurization methods often are used in combination.
Since the pre-reformation is a steam reforming process at low temperatures, special catalysts are required, in order to provide for sufficiently high reaction rates. In general, this is achieved by means of commercially available catalysts which have a high nickel content. Since such catalysts in the activated state are pyrophoric, i.e self-igniting, in air, they are supplied in an oxidized, passivated state and are incorporated into the pre-reformer in this state. During the start-up of the pre-reforming stage by the methods described in the prior art, the pre-reforming catalyst therefore must be transferred into the reduced, activated state by charging the same with a suitable reducing agent, mostly hydrogen, before the feedstocks are supplied to the pre-reforming stage. The unexamined German application DE 1545440 A describes the production of a sulfur-resistant reforming catalyst and its activation with hydrogen. It is disadvantageous here that the hydrogen required for this purpose must be supplied to the pre-reforming stage from an independent hydrogen source, since hydrogen inherent to the process is not available yet during start-up. Suitable possibilities include the delivery of hydrogen by means of a pipeline or the stockage of hydrogen in pressure tanks In both cases it is expedient to recirculate non-converted hydrogen to the pre-reforming catalyst by means of a cycle compressor. Alternatively, hydrogen for the start-up can be produced by means of a separate reforming plant, for example in miniature construction. In all these cases, however, it is unsatisfactory that the external hydrogen required for the start-up either must be supplied, stored or produced at high cost, wherein the technical apparatuses required for this purpose only are required for the start-up of the pre-reforming stage with new catalyst packing, which under typical operating conditions is effected at an interval of several years. The transport and storage of the hydrogen in the compressed state in addition involves a considerable hazard potential.
Furthermore, the use of methanol in connection with the start-up of reforming plans has been described already in the prior art. The European patent application EP 0936182 A2 for example describes a method for starting up an autothermal reformer, in which a methanol/water mixture is preheated and then charged to a methanation reactor in which the methanol is split into hydrogen, carbon oxides and small amounts of methane. The cracking gas obtained subsequently is charged to an autothermal reformer, where it serves for heating and at the same time for activating the catalyst contained in the autothermal reformer. It is disadvantageous that the document EP 0936182 A2 does not disclose an executable technical teaching for the start-up of a pre-reforming stage, in particular a pre-reforming stage in an integrated reforming method.